Because of the crowded electromagnetic spectrum used for communications, it is useful to, in effect, squeeze extra communications channels into an already established communications band or system. In the microwave field, extra data channels are accommodated in an FDM-FM voice carrier known as a Data Under Voice service, which is provided by AT&T. Other technologies narrowing channels and allowing closer channel spacing and more capacity have been found to be valuable technological advancements. In addition, there is provided apparatus for multiplexing a number of speech and low speed data channels on a single data multiplex system.
In such techniques as above, technical complexity is required to add the additional capacity without diminishing service on the previous or existing system. This includes, of course, non-interference with the existing channels.